1. Field of the Invention
The present invention relates to a semiconductor laser device having a current stopping layer for confining current. The present invention also relates to a short-wavelength laser light source which converts a laser beam emitted from a semiconductor laser device having a current stopping layer for confining current, into a second harmonic laser beam.
2. Description of the Related Art
Generally, semiconductor laser devices used as a light source in information processing or printing equipment are required to efficiently operate with low-level current. In a conventional semiconductor laser device, which is disclosed, for example, in the registered Japanese patent No. 2746131, a current confinement region including a reverse bias pn junction is provided so that current is injected into only a very small region of an active layer. This semiconductor laser device basically includes the active layer formed over a substrate, and a current confinement structure realized by p-type and n-type layers being formed above the active layer and including a current stopping layer which has an opening for current injection into only a predetermined stripe region of the active layer.
FIG. 6 is a vertical cross-sectional view of a typical example of the above semiconductor laser device. In the semiconductor laser device of FIG. 6, an n-type InGaP lower cladding layer 11xe2x80x2, semiconductor multiple layers 12xe2x80x2, and a p-type InGaP first upper cladding layer 13xe2x80x2 are formed on an n-type GaAs substrate 10xe2x80x2, where the semiconductor multiple layers 12xe2x80x2 include an i-type InGaAsP barrier layer, an i-type InGaAs quantum-well active layer, and an i-type InGaAsP barrier layer.
On the p-type InGaP first upper cladding layer 13xe2x80x2, an n-type InGaP current stopping layer 31xe2x80x2 and a p-type AlGaAs second upper cladding layer 23xe2x80x2 are formed so that the n-type InGaP current stopping layer 31xe2x80x2 exists on each side of the p-type AlGaAs second upper cladding layer 23xe2x80x2, and a current confinement structure is realized by the n-type InGaP current stopping layer 31xe2x80x2 and the p-type InGaP first upper cladding layer 13xe2x80x2. That is, the n-type InGaP current stopping layer 31xe2x80x2 has an opening filled with the p-type AlGaAs second upper cladding layer 23xe2x80x2, and a reverse bias state is realized by pn junctions between the n-type InGaP current stopping layer 31xe2x80x2 and the p-type InGaP first upper cladding layer 13xe2x80x2.
In addition, a p-type AlGaAs third upper cladding layer 24xe2x80x2, a p-type GaAs contact layer 14xe2x80x2, an insulation film 15xe2x80x2, and a p electrode 16xe2x80x2 are formed in this order on the n-type InGaP current stopping layer 31xe2x80x2 and the p-type AlGaAs second upper cladding layer 23xe2x80x2. Further, an n electrode 17xe2x80x2 is formed on the lower surface of the n-type GaAs substrate 10xe2x80x2.
However, when the current confinement structure including the reverse pn junctions is provided, the pn junctions generate parasitic capacitance. Therefore, when the semiconductor laser device is modulated at high speed, the high-frequency components pass through the pn junctions, and thus high-frequency modulation is impossible.
In addition, when the semiconductor laser device having the above problem is used in a short-wavelength laser light source in combination with an optical wavelength conversion element which converts a laser beam emitted from the semiconductor laser device, into a second harmonic laser beam having a blue or green wavelength, it is difficult to use the short-wavelength laser light source for image recording or the like.
Further, when a semiconductor laser device used in reading data from an optical disk or the like is driven at high frequency for reducing noise, high-frequency components pass through the pn junctions, and the current is not efficiently injected into the active layer.
In FIG. 6, an equivalent circuit of the semiconductor laser device is also diagrammatically indicated. As illustrated in FIG. 6, it is considered that the semiconductor laser device of FIG. 6 has as resistance components an ohmic resistance R1 in the p electrode 16xe2x80x2, a resistance R2 in the active layer, and resistances R3 and R4 in a distributed constant circuit which represents influences of the spread of the active layer in the lateral directions. In addition, the semiconductor laser device of FIG. 6 has as capacitance components a capacitance C1 existing between the p electrode 16xe2x80x2, the insulation film 15xe2x80x2, and the p-type GaAs contact layer 14xe2x80x2, capacitances C2 and C3 generated by the pn junctions at the upper and lower boundaries of the n-type InGaP current stopping layer 31xe2x80x2, a capacitance C4 generated by the junctions of the active layer, and a capacitance C5 in the above distributed constant circuit.
The parasitic capacitances C2 and C3 generated by the pn junctions at the upper and lower boundaries of the n-type InGaP current stopping layer 31xe2x80x2 become most dominant in operation with high-speed modulation, and are the major cause of the damage to the high-frequency characteristics. In particular, the areas of the pn junctions almost correspond to the area of the semiconductor laser device. In addition, viewed as an electric circuit, the pn junctions extend in parallel with the active layer. Therefore, high-frequency components can pass through the current stopping layer, and the current is not efficiently injected into the active layer.
In order to solve the above problem, Japanese Patent Publication No. 5(1993)-9951 discloses a technique for reducing parasitic capacitance existing in a current stopping layer in a buried heterostructure semiconductor laser device, which is widely used for oscillation at the wavelength of 1.3 micrometers or greater. As illustrated in FIG. 7, the semiconductor laser device has a structure in which an active layer 201 is formed above an n-type InP substrate 200, and both sides of the active layer are etched off and filled with an n-type current stopping layer 205. In addition, a pair of trenches 208 having such a depth as to reach the substrate 200 are formed on both sides of the active layer 201 so that parasitic capacitance existing in the current stopping layer 205 is reduced. Further, in FIG. 7, reference numeral 202 denotes a p electrode, 203 denotes an insulation film, 204 denotes a p-type InGaAs contact layer, and 207 denotes an n electrode.
The above technique is very useful for reducing parasitic capacitance in the current stopping layer 205 which extends through the entire area of the semiconductor laser device. However, the above structure can be formed mainly in semiconductor laser devices made of InP-based materials. In particular, from the viewpoint of the production process and reliability, the above structure cannot be formed in semiconductor laser devices made of materials which can realize oscillation at a short wavelength of 1 micrometer or smaller. The semiconductor laser devices which oscillate at a wavelength of 1.3 micrometers or greater are made of InP/InGaAsP materials, and the etching characteristics of the constituent materials of the structure of FIG. 7 are similar. That is, the structure of FIG. 7 can be realized because the formation of the trenches as illustrated in FIG. 7 is easy. On the other hand, the semiconductor laser devices which oscillate at a short wavelength of 1 micrometer or smaller are made of various materials as GaAs/AlGaAs/InGaP/InGaAsP/AlGaInP, and the etching characteristics of these materials are different. Therefore, formation of the trenches as illustrated in FIG. 7 is not easy in the semiconductor laser devices which oscillate at a short wavelength of 1 micrometer or smaller.
The registered Japanese patent No. 2746131 also discloses another technique for reducing parasitic capacitance. In the registered Japanese patent No. 2746131, this technique is applied to a semiconductor laser device having the construction as illustrated in FIG. 8, in which an n-type AlGaInP cladding layer 310xe2x80x2, a multiple-quantum-well active layer 309xe2x80x2, a p-type AlGaInP first cladding layer 308xe2x80x2, a p-type GaAs contact layer 306xe2x80x2, an insulation film 305xe2x80x2, and a p electrode 304xe2x80x2 are formed on an n-type GaAs substrate 311xe2x80x2. In addition, in FIG. 8, reference numeral 302xe2x80x2 denotes a ridge stripe, 303xe2x80x2 denotes a p-type InGaP protection layer, and 312xe2x80x2 denotes an n electrode. According to this technique, an n-type GaAs current stopping layer 307xe2x80x2 is located above the multiple-quantum-well active layer 309xe2x80x2, and a pair of trenches 301xe2x80x2 having such a depth as to reach the lower boundary of the n-type GaAs current stopping layer 307xe2x80x2 are formed by etching.
According to the above technique, only two layers are etched. Therefore, the etching is not difficult. However, the capacitance C4 generated by the junctions of the active layer as illustrated by the equivalent circuit in FIG. 6 is not reduced. Thus, the reduction of the parasitic capacitance is insufficient, and therefore the operation speed of the semiconductor laser device is limited.
An object of the present invention is to provide a semiconductor laser device which can oscillate at a wavelength of 1 micrometer or smaller, and has an improved high-frequency characteristic.
Another object of the present invention is to provide a short-wavelength laser light source in which a wavelength-converted laser light can be modulated at high speed.
(1) According to the first aspect of the present invention, there is provided a semiconductor laser device including: a substrate; a semiconductor laser device comprising: a substrate; an active layer formed above the substrate; a current confinement structure which is realized by p-type and n-type layers being formed above the active layer and including a current stopping layer which has an opening for allowing current injection into only a predetermined stripe region of the active layer; a semiconductor layer formed above the current confinement structure; a pair of trenches formed on both sides of the opening along the predetermined stripe region so as to extend from the semiconductor layer through the current stopping layer to at least the active layer; an insulation film formed on the semiconductor layer except that an area of the semiconductor layer located right above the predetermined stripe region is not covered by the insulation film; and an electrode formed on the area of the semiconductor layer.
Preferably, the semiconductor laser device according to the first aspect of the present invention may also have one or a combination of the following additional features (i) to (iv).
(i) The pair of trenches may be separated by an interval of 100 micrometers or smaller.
(ii) The electrode may have at least one area each protruding in the lateral direction and being used for wire bonding.
(iii) The substrate may be made of GaAs, the active layer may be a quantum-well active layer made of an InGaAs material, and the semiconductor laser device may emit laser light having a wavelength within the range from 0.9 to 1.2 micrometers.
(iv) The substrate may be made of GaAs, the active layer may be made of an InGaP or AlGaInP material. In this case, the active layer is, for example, a quantum-well active layer, and the oscillation wavelength of the semiconductor laser device is, for example, within the range from 0.63 to 0.68 micrometers.
(2) According to the second aspect of the present invention, there is provided a short-wavelength laser light source comprising the semiconductor laser device according to the first aspect of the present invention and an optical wavelength conversion element, where the semiconductor laser device emits a fundamental harmonic laser beam; and the optical wavelength conversion element converts the fundamental harmonic laser beam into a second harmonic laser beam.
(3) The advantages of the present invention are explained below.
(i) In the semiconductor laser device according to the first aspect of the present invention, a pair of trenches are formed on both sides of the opening along the predetermined stripe region so as to extend from the semiconductor layer through the current stopping layer to at least the active layer. Therefore, the portions of the semiconductor layers outside the pair of trenches are electrically insulated from the portion of the semiconductor layers inside the pair of trenches, and therefore the pn junctions generated at the upper and lower boundaries of the current stopping layer inside the pair of trenches are reduced compared with the pn junctions generated at the upper and lower boundaries of the current stopping layer in the conventional semiconductor laser devices. Therefore, in the semiconductor laser device according to the first aspect of the present invention, it is possible to reduce the parasitic capacitance generated at the upper and lower boundaries of the current stopping layer.
In addition, in the semiconductor laser device according to the first aspect of the present invention, an insulation film is formed on the semiconductor layer located above the current confinement structure except that an area of the semiconductor layer located right above the predetermined stripe region is not covered by the insulation film, and the electrode is formed on the area of the semiconductor layer located right above the predetermined stripe region. Therefore, when the area of the semiconductor layer on which the insulation film is not formed is minimized, and the electrode is formed after the formation of the insulation film, the area of the semiconductor layer in contact with the electrode can be effectively limited (i.e., minimized) to the area of the semiconductor layer located right above the predetermined stripe region. Thus, the areas of the pn junctions which cause the parasitic capacitance can be minimized, and the parasitic capacitance can be further reduced.
As explained above, in the semiconductor laser device according to the first aspect of the present invention, the parasitic capacitance can be remarkably reduced. Therefore, the high-frequency characteristic can be greatly improved.
(ii) When the interval between the pair of trenches is 100 micrometers or smaller, the effect of reducing the parasitic capacitance is particularly enhanced. Details of the enhancement of the effect of reducing the parasitic capacitance in an embodiment of the present invention are explained later with reference to FIG. 3.
(iii) When the electrode has at least one area protruding in the lateral direction and being used for wire bonding, the width of the electrode except for the at least one area used for wire bonding can be reduced, and therefore the total area of the electrode can be minimized. Thus, the parasitic capacitance can be further reduced.
(iv) The short-wavelength laser light source according to the second aspect of the present invention uses the semiconductor laser device according to the first aspect of the present invention as a light source which emits a fundamental harmonic laser beam, and the semiconductor laser device according to the first aspect of the present invention has a greatly improved, high-frequency characteristic. Therefore, it is possible to obtain a second harmonic laser beam which has a short wavelength and can be modulated at high speed.